PROJECT SUMMARY Testicular germ cell tumor (TGCT) is the most common cancer in men 15-45 years old, and among adult cancers results in the greatest years of life lost. Among children with gonadal/genital atypia (the applicant?s specialty), the lifetime risk of gonadal germ cell tumor is up to 50%. However, which genes predispose to these tumors, and thus would be targets for precision-medicine-based prevention and treatment approaches, remain mostly unknown. The applicant recently identified both the first Mendelian gene predisposing to TGCT, and new genome-wide association hits for TGCT. In the research project proposed here we will determine the mechanisms through which these genomic variations impair cell-autonomous germ cell specification and/or epigenetic reprogramming (Aim 1) and gonadal niche formation (Aim 2). I hypothesize that alleles which predispose to TGCT impair specific stages of germ cell development. These aims will be accomplished using innovative modeling of human germ cells with induced pluripotent stem cells (iPSCs), and a genetic biobank of ~100,000 children with phenotypic data by which to identify those with gonadal/genital atypia and risk for TGCT. This data will advance our understanding of TGCT predisposition and initiation, and will expand our knowledge of typical gonadal development. The results from this study will provide rationale for future precision cancer prevention and treatment strategies. This proposal describes a five-year plan for the applicant to develop an independent research career as an academic oncogeneticist focused on germ cell tumor predisposition and prevention. The applicant, Dr. Pyle, is an attending pediatric geneticist at Children?s Hospital of Philadelphia (CHOP) with PhD training in basic molecular biology and precision medicine. Her research focuses on identifying germline genetic features that predispose to TGCT, and understanding their mechanisms of action. The goals for this award are to further develop the skills required for a successful career as an independent investigator, including expertise in bioinformatic analysis and handling of large genetic and phenotypic data sets, modeling of human tissue with iPSCs, and cancer biology. The mentors for this award, Drs. Hakon Hakonarson and Katherine L. Nathanson, are internationally recognized leaders in pediatric genetic discovery and genetic predisposition to cancer, respectively. Dr. Pyle will be supported by a mentorship committee comprising leaders in oncology, statistical genetics, and gonadal development. Dr. Pyle will also benefit from the unparalleled resources and mentorship available at CHOP and the University of Pennsylvania.